Offshore Tanzania Research Articles

Seismic stratigraphy and petroleum prospectivity in the Northern Rovuma Basin, offshore Tanzania

The upper slope area of the Northern Rovuma Basin has been poorly studied and little is known about its development and petroleum prospectivity. Interpretation of wellbore, 2D and 3D seismic reflection data in this area has allowed identification of seismic features reflecting key factors for understanding the development of the Cretaceous-Holocene stratigraphy and distribution of petroleum system elements. Our results show that infill of sediment into the basin was due to an interplay of three major factors. These are the sea level variations, extensional tectonics and sediment gravity flows. The Miocene and Pleistocene-Holocene tectonic events triggered gravity flows that supplied sediments to the basin. The resultant deposits include slides, slumps, debrites and turbidites some of which contain potential reservoirs that are interpreted to have been charged by the Permo-Triassic Karoo shales and Cretaceous-Cenozoic source rocks. These reservoirs are contained in both stratigraphic and structural traps with localized combinations in some places, and are encased by deep marine shales. Some of the potential reservoirs are positively inverted and compartmentalized, and they contain several bright and flat spots suggesting hydrocarbon accumulations within the Miocene and Oligocene intervals. It has also been revealed that major gravity flows and bottom currents, that supplied and distributed sediment to the basin, were confined in three long-lived channelized systems that were initiated during the Cretaceous (due to rapid regression), and the Oligocene and Pleistocene periods (due to rapid uplift). The channels migrated toward the southeast, with some shift toward the northeast, and had overall northeast traverse direction perpendicular to the shoreline. The reported gravity flows eroded previously deposited sediment in most places causing several unconformities in the whole Cretaceous-Holocene stratigraphy. Seismic stratigraphic interpretation and facies analysis have proved useful in the identification of key petroleum system elements and improve understanding of sedimentary fill evolution of the study area.

Late Cenozoic mass transport deposits in the offshore Tanzania continental margin

Sediment mass movements and their associated Mass Transport Deposits (MTDs) have been widely studied due to their economic and geohazard potentials. This study combines 2D and 3D seismic reflection data with a seismic facies approach and attribute analysis to reveal the presence and distribution of different MTDs in the southernmost region offshore Tanzania. Results of seismic facies analysis show that the study area contains different Late Cenozoic MTDs. The MTDs have limited petroleum reservoir potential and include slides, slumps, debris flow deposits and occasional turbidites. The formation of these MTDs was caused by tectonic events associated with the development of the East African Rift System. Seismic attribute maps have shown the locations of channels, remnant blocks, headwall scars, and grooves confirming downslope sediment mass mobilization. The seabed attribute maps have shown areas where recent mass mobilizations were initiated. Some of these areas coincide with faults which have dissected the seabed, forming potential future gravity failure sites. Other future gravity failure sites include channel banks and slope edges, which may be present over the whole Tanzania continental margin. Sediment mass movements may be catastrophic, and therefore future infrastructure installations in the area must involve detailed mapping of the seabed to assess geohazard risks and act accordingly.

Cretaceous deposits of the northern part of the southern offshore Tanzania: depositional system, seismic manifestation, well data expressions and their implications on petroleum system elements

Cretaceous deposits of the northern part of the southern offshore Tanzania: depositional system, seismic manifestation, well data expressions and their implications on petroleum system elements

Basin development and petroleum prospectivity of the hybrid turbidite-contourite system in the east Pande area, offshore Tanzania

2D seismic reflection and wellbore data were used to investigate the structure and stratigraphy of the East Pande block, offshore Tanzania, allowing an assessment to be made of basin development and the occurrence of petroleum system elements. Interpretation of the 2D seismic data showed that patterns of sedimentation were controlled by the Cretaceous- Quaternary oceanic circulation (including bottom currents) together with sea level changes and the basin-ward input of detrital material from the Tanzania mainland, resulting in the development of at least five reservoir-seal combinations. The identified reservoir rocks include hybrid turbidite-contourite deposits together with channel sands and their associated levee successions which have moderate porosity and a low shale volume. Regionally, the hybrid turbidite-contourite deposits have not been studied in detail; their presence in the East Pande area may be of interest to both local and regional researchers. Potential structural traps in the study area include faults and local folds; another trapping mechanism is provided by stratigraphic pinch-outs which are characterized in seismic data by flat spots indicative of hydrocarbon accumulations. Deep-rooted faults and erosive channel bases are interpreted to have allowed petroleum migration from underlying source rocks to shallower reservoir intervals. The mapped faults in the study area have N–S to NNW-SSE trends which conform to the Permo-Triassic fault systems of the coastal Tanzania basin. This suggests that the locations and orientations of younger faults was controlled by the pre-existing structural grain linked to Karoo (Permo-Triassic) faulting. The results of this study suggest that the East Pande area offshore Tanzania may have significant petroleum prospectivity.

Architecture, structural and tectonic significance of the Seagap fault (offshore Tanzania) in the framework of the East African Rift

AbstractThe Southeastern portion of the East African Rift System reactivates Mesozoic transform faults marking the separation of Madagascar from Africa in the Western Indian Ocean. Earlier studies noted the reactivation of the Davie Fracture Zone in oceanic lithosphere as a seismically active extensional fault, and new 3D seismic reflection data and exploration wells provide unprecedented detail on the kinematics of the sub‐parallel Seagap fault zone in continental/transitional crust landward of the ocean‐continent transition. We reconstruct the evolution of the seismically active Seagap fault zone, a 400‐km‐long crustal structure affecting the Tanzania margin, from the late Eocene to the present day. The Seagap fault zone is represented by large‐scale localized structures affecting the seafloor and displaying growth geometries across most of the Miocene sediments. The continuous tectonic activity evident by our seismic mapping, as well as 2D deep seismic data from literature, suggests that from the Middle‐Late Jurassic until 125 Ma, the Seagap fault acted as a regional structure parallel to, and coeval with, the dextral Davie Fracture Zone. The Seagap fault then remained active after the cessation of both seafloor spreading in the Somali basin and strike‐slip activity on the Davie Fracture Zone, till nowaday. Its architecture is structurally expressed through the sequence of releasing and restraining bends dating back at least to the early Neogene. Seismic sections and horizon maps indicate that those restraining bends are generated by strike‐slip reactivation of Cretaceous structures till the Miocene. Finally based on the interpretation of edge‐enhanced reflection seismic surfaces and seafloor data, we shows that, by the late Neogene, the Seagap fault zone switched to normal fault behaviour. We discuss the Seagap fault's geological and kinematic significance through time and its current role within the microplate system in the framework of the East African rift, as well as implications for the evolution and re‐activation of structures along sheared margins. The newly integrated datasets reveal the polyphase deformation of this margin, highlighting its complex evolution and the implications for depositional fairways and structural trap and seal changes through time, as well as potential hazards.

Bottom Current Modification of Turbidite Lobe Complexes

Submarine lobes form at the distal end of sediment gravity flow systems and are globally important sinks for sediment, anthropogenic pollutants and organic carbon, as well as forming hydrocarbon and CO2 reservoirs. Deep-marine, near bed or bottom currents can modify gravity flow pathways and sediment distribution by directly interacting with the flow or by modifying seafloor morphology. Deciphering the nature of gravity- and bottom currents interaction, particularly in ancient systems, remains a challenge due to the lack of integrated datasets and the necessary oceanographic framework. Here we analyse high-resolution 3D seismic reflection and core data from the Upper Cretaceous interval offshore Tanzania to reveal the interaction of turbidite lobes with fine-grained sediment waves and contourite drift deposits. Contourite drift morphology governs the large-scale confinement style and shape of lobes that range from frontally confined and crescent shaped, to laterally confined and elongated, to semi-confined lobes. Core data reveals massive to cross-laminated high density turbidites in the lobe axis position that show no direct interaction between gravity flows and contour currents. Lobe off-axis and fringe deposits consist of parallel- and ripple-laminated, low density turbidites, which are inter-bedded with bioturbated, muddy siltstones that represent the toes of contourite drifts. Starved ripples, and streaks of up to fine-grained sandstone above individual turbidite beds indicate reworking by bottom currents. This facies distribution reflects the temporal interaction of quasi-steady bottom currents and turbidity currents that interact with the topography and build lobes over short periods of time. Frontally confined turbidity currents form lobes in a fill-and-spill fashion, in which the confinement of turbidity currents causes rapid deposition and obscures any bottom current signal. Lateral confinement causes increased turbidity current runout length, and promotes the development of lobe fringes with a high proportion of bottom current reworked sands. During times when sediment gravity flows are subordinate, contourites accumulate on top of the lobe, confining the next flow and thus modifying the overall stacking pattern of the lobe complex. Although sediment volumes of these bottom current modified lobe complexes are comparable to other deep-marine systems, bottom currents considerably influence facies distribution and deposit architecture.

Characterization of the accommodation zones along restraining and releasing bends from analogue modelling simulating the seagap fault, off-shore Tanzania

The study aims to understand geometry and evolution of accommodation zones along restraining and releasing bends in strike slip systems which is often complex and characterized by sharp change in polarity, resulting in a difficult characterization of traps location. A batch of sandbox modelling experiments was performed using a layered scaled sand material. The setup parameters of the wooden baseplates used in the first two experiments (SS1 and SS2) are basement offset of 6.5 cm/3 cm, 90°/90° stepovers in releasing and restraining bends of total 96 cm length and 25 cm width. The third experiment (SS3) was performed with basement offset of 6.5 cm/3 cm, 156°/126° stepovers in releasing and restraining bends of total 96 cm length and 25 cm width. The experiments were performed with special attention to the role of syn-kinematic sedimentation and the pre-existing structure of the basement. A sequence of pop-up and pull-apart structures was produced. Along the main structures, complex fault trend was compatible with a sinistral riedel distribution and a counterclockwise rotation of pre-existent elements. Change of fault's polarity at depth and dip direction flipping of high angle faults in accommodation zones were clearly observed. Even more, progressive propagation of the pop-up structures produces a spectacular inversion of adjacent extensional structures. Comparing results of the model with the Seagap fault zone, it is possible to put in evidence similar fault orientation and distribution. Internal structures developed were strongly influenced by syn-kinematic sedimentation as observed by characteristic helicoidal shape of growth faults. The analogue models clearly describe how boundary faults of the pull-apart basin along a paired bend, initially characterized by a normal kinematic can be subsequently inverted in the later stages, which can potentially have a strong impact on the petroleum system.

Unconventional hydrocarbon resources in Tanzania: insight from geology and previous exploration of conventional resources

Unconventional oil and gas exploration has been developing rapidly in recent decades, while Tanzania has been paid very little consideration. This review aimed to infer these unconventional hydrocarbon resources in some basins, mostly the intracratonic Karoo basins and one coastal basin (Ruvu basin), from a piece of the very limited information available from the previous exploration of conventional resources. The presence of thick dark carbonaceous shales of Makarawe formation at Ruvu basin with TOC% 2.4–4.0% and a thickness between 107 and 698 m capped with non-porous oncolitic limestone and the gas shows encountered at an interval of 107 m of Makarawe shales at Makarawe-1 well makes Ruvu basin as important play for unconventional shale gas. The intracratonic Karoo basins have a thick deposit of Karoo sediments interbedded with coal deposits. Coal bedded methane has been encountered in a deep coal exploration borehole at Songwe-Kiwira coalfield. Geochemical analysis of outcrop samples from Namwele, Mbamba Bay, Njuga, and Mhukuru coalfields shows that the interbeds of coal-carbonaceous shales/mudstone have the rich organic matter of type III or both II and III and trace of oil on samples from Ketewaka and Ngaka coalfield make these sub-basins to be considered as potential plays for shale oil/gas. The presence of probable shale gas resources ranging from 50 to 200 trillion cubic feet at the Selous basin increases Karoo basins’ potentiality in exploring unconventional gas resources. In addition to the discovered conventional gas resource in offshore Tanzania, any unconventional resources will impact the country’s economic growth.

Hybrid turbidite-drift channel complexes: An integrated multiscale model

Abstract The interaction of deep-marine bottom currents with episodic, unsteady sediment gravity flows affects global sediment transport, forms climate archives, and controls the evolution of continental slopes. Despite their importance, contradictory hypotheses for reconstructing past flow regimes have arisen from a paucity of studies and the lack of direct monitoring of such hybrid systems. Here, we address this controversy by analyzing deposits, high-resolution seafloor data, and near-bed current measurements from two sites where eastward-flowing gravity flows interact(ed) with northward-flowing bottom currents. Extensive seismic and core data from offshore Tanzania reveal a 1650-m-thick asymmetric hybrid channel levee-drift system, deposited over a period of ∼20 m.y. (Upper Cretaceous to Paleocene). High-resolution modern seafloor data from offshore Mozambique reveal similar asymmetric channel geometries, which are related to northward-flowing near-bed currents with measured velocities of up to 1.4 m/s. Higher sediment accumulation occurs on the downstream flank of channel margins (with respect to bottom currents), with inhibited deposition or scouring on the upstream flank (where velocities are highest). Toes of the drift deposits, consisting of thick laminated muddy siltstone, which progressively step back into the channel axis over time, result in an interfingering relationship with the sandstone-dominated channel fill. Bottom-current flow directions contrast with those of previous models, which lacked direct current measurements or paleoflow indicators. We finally show how large-scale depositional architecture is built through the temporally variable coupling of these two globally important sediment transport processes. Our findings enable more-robust reconstructions of past oceanic circulation and diagnosis of ancient hybrid turbidite-drift systems.

Impact of the East African Rift System on the routing of the deep‐water drainage network offshore Tanzania, western Indian Ocean

AbstractThe East African Rift System (EARS) exerted a major influence on river drainage basins and regional climate of east Africa during the Cenozoic. Recent studies have highlighted an offshore branch of the EARS in the western Indian Ocean, where the Kerimbas Graben and the Davie Ridge represent its sea floor expression. To date, a clear picture of the impact and timing of this EARS offshore branch on the continental margin of the western Indian Ocean, and associated sediment dispersal pathways, is still missing. This study presents new evidence for four giant canyons along the northern portion of the Davie Ridge offshore Tanzania. Seismic and multibeam bathymetric data highlight that the southernmost three canyons are now inactive, supra‐elevated relative to the adjacent sea floor of the Kerimbas Graben and disconnected from the modern slope systems offshore the Rovuma and Rufiji River deltas. Regional correlation of dated seismic horizons, integrated with well data and sediment samples, proves that the tectonic activity driving the uplift of the Davie Ridge in this area has started during the middle‐upper Miocene and is still ongoing, as suggested by the presence of fault escarpments at the sea floor and by the location and magnitude of recent earthquakes. Our findings contribute to placing the Kerimbas Graben and the Davie Ridge offshore Tanzania in the regional geodynamic context of the western Indian Ocean and show how the tectonics of the offshore branch of the EARS modified the physiography of the margin, re‐routing the deep‐water drainage network since the middle Miocene. Future studies are needed to understand the influence of changing sea floor topography on the western Indian Ocean circulation and to evaluate the potential of the EARS offshore tectonics in generating tsunamigenic events.

Intergrated Seismic Stratigraphic and Structural Analysis of the Songo Songo Gas-Field, Shallow Offshore Tanzania, Using Seismic and Well Data

Seismic stratigraphy and structural analysis of seismic data, when combined with wireline data, provides vital information for hydrocarbon exploration in a prospective basin. These techniques were employed in the Songo Songo shallow coastal basin, southern Tanzania to determine stratigraphic setting, trapping mechanism, reservoirs zones. Suites of well logs from two wells (AA-1 and BB-5) and 2-D seismic data was obtained from TPDC in the study area. Lithologic interpretation and well correlation were carried out using the well log suites. Stratigraphic analysis was carried out with the well logs and 2D seismic data by using principles of sequence stratigraphy while structural interpretation was done with aid of the seismic data to produce polar plots for the different structural trends. The lithology was dominated by sand, shale, and limestone. Five sequence boundaries (SB); SB1 (Albian), SB2 (base of Coniacian-Early Campanian), SB3, (Middle Eocene), SB4 (Late Miocene) and SB5 (Quaternary)) and three maximum flooding surfaces (MFS); MFS1-(Early Cretaceous), MFS2, (Late Cretaceous), MFS3-(Early Eocene) were identified. Three main normal faulting systems (Jurassic faults, active during the Jurassic rifting phase of Madagascar, Neogene faults that occurred during Neogene east African rifting and reactivated faults which mostly were Jurassic fault reactivated by east African rifting) of NNW-SSE were recognized in the study area. The structural interpretation reveal that the gas field is dominated by normal faults that occur in the upper and lower part of Songo Songo suggesting two phases of deformation prior to development of the field. The main reservoir is developed in the Neocomian and Albian sandstones, and capped by Jurassic faults and Wami (Formation) overlying rocks. The gas is sourced from Jurassic shale known as Mtumbei Formation and stored at the main reservoir developed in the Lower Cretaceous aged Neocomian sandstone known as Kipatimu Formation, sealed by high pressured shale of upper Cretaceous known as Wami Formation. This study shows that deposition of sediment occurs in the NW – SE direction, with the thinning of sediments thickness towards well BB-5 while the stratigraphic sections show that the horizons are laterally continuous and are being strongly affected by tectonic events.

Petrophysical Analysis of the Mpera well in the Exploration Block 7, Offshore Tanzania: Implication on Hydrocarbon Reservoir Rock Potential

The present study provides evaluation and estimation of petrophysical parameters and assessment of lithology and their thicknesses in order to characterize present reservoir rocks at Mpera well located in Exploration Block 7, deep offshore Tanzania. To achieve the objectives the wire-line logs, Techlog program was used for assessment, analysis, computation and interpretations of petrophysical parameters and results were integrated through interpretation of well logs. The results from wire-line logs reveal three (3) non hydrocarbon-bearing reservoir rocks i.e., Mpera splay (sandstone), Mpera deep sand 1 (sandstone and limestone) and Mpera deep sand 2 (sandstone and limestone) with gross thickness of 94.335 m, 28.905 m and 12.967 m respectively. The average permeability values of the reservoir rocks were 9.47 mD, 6.45 mD and 4.67 mD, while average porosity values were 14.57%, 17.4% and 16.75%, with average volume of shale 25.7%, 23.5% and 9.7% at Mpera splay, Mpera deep sand 1 and Mpera deep sand 2 respectively. These results signify poor permeability; good porosity and good quality reservoir in terms of volume of shale. Fluid type defined in the reservoirs was basically water. High water saturation (90.6% - 97.7%) in the reservoir zones of the Mpera well indicates that the proportion of void spaces occupied by water is high, thus, indicating less than 10% hydrocarbon saturation. The findings indicate that Mpera well reservoir rocks are of low quality with non-hydrocarbon bearing such that it is not potential for hydrocarbon production.

Tectono-Sedimentary Evolution of the Offshore Hydrocarbon Exploration Block 5, East Africa: Implication for Hydrocarbon Generation and Migration

Sedimentary deposits in Block 5, offshore Tanzania basin have been imaged using two-dimensional (2D) seismic data. The seismic data and well data reveal four tectonic units representing different tectonic events in relation to structural styles, sedimentation and hydrocarbon potential evolved in Block 5. Results show that during Early to Late Jurassic, Block 5 was affected by the break-up of Gondwana and the drifting of Madagascar as evidenced by patterns of sediments and structural features. The chaotic and discontinuous reflectors are characteristics features on the sediments pattern indicating a possible transitional setting following the breakup of Gondwana. From the Late Cretaceous, Block 5 sits in more stable subsiding sag as the consequence of the high thermal subsidence. The period displayed continuous parallel reflectors with few markable faults. This was followed by the late post rift sedimentation that occurred after Middle Eocene Unconformity characterized by high wavy and sub parallel reflectors. The evolution of Block 5 through major tectonic events reveals a more complete petroleum system towards the south. Thus, Block 5 responded in both space and time to a complex interplay between tectonics and sedimentation. This indicates that structural styles and associated features are potential control for hydrocarbon generation and migration.

Introduction to the thematic set: Tectonics and petroleum systems of East Africa

This paper provides an overview of a series of papers to be published within two issues of Petroleum Geoscience in 2018 expressing the theme ‘tectonics and petroleum systems of East Africa’. These papers partly result from the Geological Society of London’s Petroleum Group conference in April 2016 on ‘East Africa; From Research to Reserves’. The theme of this conference highlighted the advances that have been made since a previous East Africa conference as a result of the recent major exploration efforts. This issue (February 2018) concentrates on the regional tectonics and include overviews of our current understanding of Permian to Mesozoic rifting (Macgregor), Tertiary rifting (Purcell), the plate tectonic model (Reeves), and of the development of the East African margin (Davison & Steel). The August 2018 issue is planned to include more basin and regional specific papers on the Kenyan rift system, offshore Somalia, offshore Tanzania and offshore Mozambique.

Madagascar's escape from Africa: A high‐resolution plate reconstruction for the Western Somali Basin and implications for supercontinent dispersal

AbstractAccurate reconstructions of the dispersal of supercontinent blocks are essential for testing continental breakup models. Here, we provide a new plate tectonic reconstruction of the opening of the Western Somali Basin during the breakup of East and West Gondwana. The model is constrained by a new comprehensive set of spreading lineaments, detected in this heavily sedimented basin using a novel technique based on directional derivatives of free‐air gravity anomalies. Vertical gravity gradient and free‐air gravity anomaly maps also enable the detection of extinct mid‐ocean ridge segments, which can be directly compared to several previous ocean magnetic anomaly interpretations of the Western Somali Basin. The best matching interpretations have basin symmetry around the M0 anomaly; these are then used to temporally constrain our plate tectonic reconstruction. The reconstruction supports a tight fit for Gondwana fragments prior to breakup, and predicts that the continent‐ocean transform margin lies along the Rovuma Basin, not along the Davie Fracture Zone (DFZ) as commonly thought. According to our reconstruction, the DFZ represents a major ocean‐ocean fracture zone formed by the coalescence of several smaller fracture zones during evolving plate motions as Madagascar drifted southwards, and offshore Tanzania is an obliquely rifted, rather than transform, margin. New seismic reflection evidence for oceanic crust inboard of the DFZ strongly supports these conclusions. Our results provide important new constraints on the still enigmatic driving mechanism of continental rifting, the nature of the lithosphere in the Western Somali Basin, and its resource potential.

Gondwana breakup: no evidence for a Davie Fracture Zone offshore northern Mozambique, Tanzania and Kenya

AbstractPlate tectonic reconstructions assume a major inactive transform fault, the Davie Fracture Zone, in the West Somali Basin, along which Madagascar is thought to have migrated southwards following Gondwana breakup in the Mesozoic. Based on the interpretation of reflection seismic data, we show that the Walu Ridge offshore Kenya and the Kerimbas Basin offshore northern Mozambique are tectonically unrelated to the southward motion of Madagascar and correlate with Late Cretaceous volcanism and inversion in Kenya and the evolution of the East African Rift System respectively. Offshore Tanzania, geophysical data do not show basement structures indicating the presence of a major transform fault. These results challenge the commonly supported transform margin concept and imply a more southerly pre‐breakup position of Madagascar within Gondwana. Opening of the West Somali Basin by SW‐propagating oblique rifting and seafloor spreading is proposed.

Zonal Isolation Through Gas Hydrates Offshore Tanzania

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 163462, ’Zonal Isolation Through Gas Hydrates Offshore Tanzania,’ by J. Volstad and T. Tveit, Statoil, and P. Aguilar, N. Hurtado, and M. Bogaerts, Schlumberger, prepared for the 2013 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 5-7 March. The paper has not been peer reviewed. Deepwater cementing becomes increasingly challenging as drilling operations move to greater water depths and more-remote locations. Before the first exploration well for an operator offshore Tanzania was spudded, an extensive cementing-operations risk analysis was performed. The risks determined were low temperatures at seabed, unconsolidated formations close to seabed, potential shallow gas, and likely presence of hydrates. To drill further sections successfully, objectives were identified as zonal isolation across the shallow-flow and hydrate zones, cementing back to seabed, and good cement around the casing shoe. Introduction Block 2 (Fig. 1) is located southeast of Tanzania’s capital, Dar es Salaam. The water depths in Block 2 vary from 1000 to 3000 m. The target is an excellent-quality sandstone gas reservoir with very large potential. The seabed consists of soft clays and silts with minor sandy intervals. Shallow hazards were identified for this block from seismic data. Preparation for the exploration phase started in 2007 when the license for Block 2 was awarded. Two exploration wells have been drilled in Block 2 offshore Tanzania to date. The wells were drilled in ultradeep water, with respective water depths of 2419 and 2627 m. Both wells were drilled successfully and have found commercial gas reserves. Well Design The well design for both wells was jetting a 36-in. conductor casing to provide sufficient well-bearing capacity. The surface casing was set deep enough to isolate all shallow zones with potential hydrates and shallow water and gas flow. The intermediate string was set above the top of the target reservoir, and the reservoir was drilled and plugged after the reservoir was evaluated. Both exploration wells were drilled vertically to total depth. The riserless section was drilled with seawater and water-based mud, while the deeper sections were drilled with a synthetic-based mud. Challenges The main challenges for the two wells were related to the great water depth (low fracture gradients and low temperatures) and the risk of encountering hydrates and potential shallow flows, which would need to be isolated by the surface casing.

East African gas-impacts for Australian LNG

Until recently, East Africa, with its complex geology and seemingly limited prospects, was the poor relation of the hydrocarbon provinces of West Africa. Since 2010, however, a string of successful exploration has resulted in offshore Kenya, Tanzania, and Mozambique, culminating in significant mergers and acquisitions and farming activities. Peter Coleman of Woodside described it as a potential game changer and a significant threat to the Australian LNG market. This extended abstract provides an overview of the basins and the discoveries, concentrating on the two most promising countries: Mozambique and Tanzania. The deals to date and the proposed LNG developments are also discussed. Also discussed is the petroleum regimes in each of these jurisdictions, the deals, the underlying title systems, the absence of regulation, and the key risks for parties transacting in that sector. An overview of applicable taxation regimes is supplied. This extended abstract then considers potential development scenarios and the relative advantages and disadvantages that East Africa has compared with Australia and the degree to which East Africa presents a threat to planned Australian projects. It is self-evident that the absence of infrastructure and modern petroleum systems of regulation challenge investment decisions.

Processes of diversification and dispersion of Rice yellow mottle virus inferred from large‐scale and high‐resolution phylogeographical studies

Phylogeography of Rice yellow mottle virus (RYMV) was reconstructed from the coat protein gene sequences of a selection of 173 isolates from the 14 countries of mainland Africa where the disease occurred and from the full sequences of 16 representative isolates. Genetic variation was linked to geographical distribution and not to host species as isolates from wild rice always clustered with isolates from cultivated rice of the same region. Genetic variation was not associated to agro-ecology, viral interference and insect vector species. Distinct RYMV lineages occurred in East, Central and West Africa, although the Central African lineage included isolates from Benin, Togo and Niger at the west, adjacent to countries of the West African lineage. Genetic subdivision at finer geographical scales was apparent within lineages of Central and West Africa, although less pronounced than in East Africa. Physical obstacles, but also habitat fragmentation, as exemplified by the small low-lying island of Pemba offshore Tanzania mainland, explained strain localization. Three new highly divergent strains were found in eastern Tanzania. By contrast, intensive surveys in Cote d'Ivoire and Guinea at the west of Africa did not reveal any new variant. Altogether, this supported the view that the Eastern Arc Mountains biodiversity hotspot was the centre of origin of RYMV and that the virus spread subsequently from east to west across Africa. In West Africa, specific strains occurred in the Inner Niger Delta and suggested it was a secondary centre of diversification. Processes for diversification and dispersion of RYMV are proposed.